Mevinolin, also known as Lovastatin is a potent HMG-CoA reductase inhibitor and as such is an effective antihypercholesterolemic agent. Patchett et al. (U.S. Pat. No. 4,351,844) have found that a 3,5-dihydrolovastatin (I) is also a potent HMG-CoA reductase inhibitor. ##STR1## Patchett, et al., supra, describe a catalytic hydrogenation process, employing palladium on calcium carbonate in absolute ethanol, for the reduction of lovastatin to a compound of structure (I). This process has also been described by Kuo et al. J. Org. Chem, 48, 1991(1983). This palladium catalyzed hydrogenation has been described as yielding the 3,5 dihydrolovastatin together with varying amounts of the 3,4-dihydro isomer as a contaminant. Kuo et al, supra, also describe an alternative procedure for the synthesis of compound I vis-vis the treatment of tertbutyldimethylsilyl-lovastatin with triethyl silane in methylene chloride followed by protolysis with trifluoroacetic acid. However, this process requires silylation-desilylation and results in low yields of the desired 3,5-dihydro reduction product.